TRANSPORTATION RESEARCH RECORD 1104

Grouting on the Run

MARTIN PRAGER

A pressure-grouting technique was used to repair a highway section damaged by erosion In the base course from water infiltrating through joints and cracks. The grouting objectives for this project were established in an effort to underseal the highway section, preventing further erosion. To accomplish this task, new techniques were created to overcome material­handling and time constraints. A rarm tractor was rebuilt and converted to a drilling machine that allowed a series of holes to be drilled simultaneously in order to drill the more than 67,000 holes estimated for this project. This equipment, with grouting plant and compressors In tow, performed assembly line opera­tions and all grouting work was completed In 80 working days. After experimenting with regular rock and carbide bits, car­bide bits were chosen for their durability and superior strength characteristics. The Texas State Department of High­ways and Public Transportation (TDHPT) developed specifica­tions and other constraints for this work. Some of the most notable included the number of hours that could be worked per day, testing of grout mix characteristics, and clean-up requirements. All short-term objectives were achieved by grouting, that is, the stabilization of the slab and undersealing. In addition the scope of the project was found to be larger than most (36 ml or 58 km), and preliminary grout quantities were hard to obtain, though Independent estimates by TDHPT were within roughly 20 percent of actual quantities of grout used. The experience gained by completion of this project was con­sidered valuable for future state highway rehabilitation pro­jects.

Pressure grouting is an effective and labor-saving means for extending the life expectancy of a highway system that is experiencing problems created by subsurface conditions. It is used by many state highway and transportation agencies to accomplish three primary functions: undersealing existing pavement, raising deflected pavement, and filling voids.

A section of Interstate highway 35E between Waxahachie and Hillsboro, Texas, was deteriorating rapidly due to increased and heavy load-bearing traffic. Moisture was infil­trating the pavement and accumulating underneath. Over time, the cycling of heavy loads and resulting hydraulic pressures had created pocket voids between the pavement and base mate­rial. To help minimize further deterioration, the Texas State Department of Highways ,and Public Transportation (TDHPT) had installed a series of subdrains. The drains were successful in removing some water from beneath the pavement; however, the continued infiltration of water combined with heavy truck loads led to a worsening pavement condition.

The grouting objectives for this project were established to undersea! the highway section to prevent further infiltration of moisture and subsequent pavement deterioration. The job

BPR Grouting and Engineering, Inc., P.O. Box 59001, 1601 Tantor Rd., Dallas, Tex. 75229.

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description did not include the raising or leveling of pavement sections, or both.

To repair this highway section the scope of work included replacing broken slab sections, undersealing the pavement, replacing the shoulders, and overlaying the entire pavement with an asphaltic concrete leveling course. The project con­sisted of approximately 4.5 million square feet (418 500 m2) of pavement. The work area for grouting was almost 2.8 million square feet (260 400 m2), an area 18 mi (29 km) of one lane northbound and 18 mi (29 km) of one lane southbound.

The concrete was 8-in. (20-cm) and 10-in. (25-cm) thick continuous reinforced pavement laid upon a 3-in. (15-cm) sta­bilized crushed limestone base course. The grout holes were to be drilled to a maximum depth of 12 in. (30 cm). This limitation was specified by the TDHPT to restrict the grout placement between the bottom of the concrete and the top of the subbase, as the primary problem was water infiltrating through joints and cracks causing erosion in the base course.

The grouting portion of this project was originally designed for 120 working days and estimated time for completing the entire project was 360 days. In order to save a paving season, the general contractor decided to push the completion date ahead 180 days. By another agreement it was established that the grouting portion would be completed in 80 working days. Some other notable restrictions in the scope of work required careful planning. Traffic could not be blocked. This meant that traffic had to move in at least one lane at all times and only 2 mi (3.2 km) of highway (in each lane) could be barricaded during work progress. Strict traffic patterns had to be followed in moving equipment and materials, and no U-turns, crossing of median strips, or parkways were allowed. Although six turn­around points were located inside the 18-mi (29-km) highway work area, logistical problems still remained on the length of supply routes. The spacing of the exits (including the beginning and ending exits) was such that it was common to travel a 25-mi (45-km) round trip to get water or materials and it could easily take more than 1 hr for each trip. The limitation to one lane for working eliminated larger off-road equipment and facilities normally available at a dam work site. For example, it is not uncommon to have silos for bulk material storage and distribution. water pipelines for grouting and clean-up, and large stationary grout plants, all located on the work site. The limited area and the requirement to completely grout more than 3,000 linear ft (900 m) per day made bulk material handling and storage infeasible. Therefore, all materials were supplied in bags and handled with forklifts and by personnel. Italy, Texas, was selected to be the storage and staging area for the job. Although several miles off the highway, it was still the most accessible site for the project.

The most difficult problem encountered posed an interesting challenge. Because extended periods of rain prevented the holes from beiI?-g pre-drilled, the Hydraulic Ram Effect, caused

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by loading and the action of water in the holes, could accelerate the deterioration of the subbase.

Mobilization at the staging area began in mid-May 1984 in anticipation of a starting date the last week of the same month.

The grout specifications called for a mixture of 3 parts pozzolan (fly ash) to 1 part cement (Type n and a super­plasticizer as needed A typical batch consisted of six bags (450 lb or 204 kg) of the pozzolan to two bags (188 lb or 85.3 kg) of portland cement Type I, and 1 oz (30 ml) of superplasticizer. The amount of water re.quired was to be sufficient to create a flow that would meet the requin:ments of the TDHPT Flow Cone Method. In this case, the average volume of water used per batch was 27 gal.

The Flow Cone Method is a field test used for determining the flow of grout mixtures by measuring the time of efflux of a specified volume of grout from a standardized flow cone. The apparatus is a simple cone device normally com;trucle<l of cast aluminum. The procedure is to moisten the inside surface of the flow cone and to place a finger over the outlet of the discharge tube while grout is introduced into the cone. A specific quantity of grout is measured when the grout surface is in contact with the point gauge. The stop watch i~ started and the finger removed simultaneously. The elapsed time is noted at the clean out of grout from the discharge tube, and the time indicated by the stop watch is the time of efflux of the grout. At least two tests were conducted for any grout mixture and the average time was used to verify compliance with specifications.

The grouting equipment consisted of one shop-made plant, mounted on a trailer, and a commercial trailer-mounted plant. The shop-made plant was a standard grout unit consisting of a high-speed colloidal mixer, agitating tank, water tank, and two Moyno pumping units. The smaller commercial plant was to serve as a standby or filler for small areas that had been passed for one reason or another. A 600 fl3/min (17 m3/min) com­pressor was used to power the air-operated equipment.

A unique method was devised to meet the challenge of drilling more than 67,000 holes estimated for the project. Nor­mally all holes are hand drilled but there are exceptions, as in this special case where time constraints were important. Conse­quently, design and reliability of the drilling machine were critical to the successful completion of the project.

A farm tractor was converted to a drilling machine that allowed a series of holes to be drilled simultaneously. Air­operated drills were attached to a carrier beam, which in turn was controlled by the tractor hydraulics. The beam was con­structed to allow rotational ability to perform varied drilling patterns along latitudinal and longitudinal axes. Preconstruc­lion experimen1ation revealed that for this project four drills driven by a 450-ft3/min (l3-m3/min) compressor were the most practical means to obtain the desired cost-effective production. The drill tractor towed the compressor and was e.quipped with a guide rod attached to the front to enable the operator to main­tain the proper spacing and alignment of the holes. The machine was able to drill four holes and move into position for the next set within 90 sec. One man was required to operate the drill machine. Another man accompanied the drill to change drill steel and rock bits when necessary. Whenever the drills ran into reinforcing steel the operator withdrew that particular drill and continued on. The assistant, using a hand drill (which was connected to the air compressor) would tl1en drill another hole adjacent to the blocked hole.

TRANSPORTATION RESEARCH RECORD 1104

The drilling methodology was established in the beginning by trial and error and also by observation of time trials when

using the e.quipment. For example, regular rock bits were used at first, but later were changed to hard carbide bits when it was observed that only 40 holes could be drilled per regular bit to about 200 holes for each carbide bit. Another problem encoun­tered at high drilling speeds was fatigue and cracking in the drill steel. The problem was solved by changing drill steel every 30 holes and rotating material to increase the longevity of the steel, reducing steel replacement by more than 70 percent.

A normal work day would begin with the drilling crew at work at 8:00 a.m. (time restriction by state), while the grout train was still being prepared for operation. About an hour later, after drilling ahead, the actual grouting procedure would begin. The Flow Cone test was used to determine the amount of water required for the first batch. It was run any time the plant started operations and at several times throughout the shift.

The grouting procedure used 100 ft (30 m) of l1/2-in. (4-cm) high-pressure grout hose connected to a rubber nozzle. The nozzle had mechanical attachments so it could also work as an expandable packer. An on-off valve was installed at the nozzle head to ensure complete control over the pumping. A pressure gauge was located on the discharge side of the pump. The necessity of grouting all the drilled holes during the same day limited the available drilling time to 7 hr each day. The drill crew spent the remaining time marking holes ahead or going behind the grouting operation to assist in cleaning up and patching holes. The holes were patched with the same grout mix, which meant placing grout from a bucket into each hole.

A flatbed truck towed the grout plant, with the air com­pressor connected to the rear of the grout plant. A water truck was used to supply the bulk water for the tank on the grout trailer and for cleaning the pavement behind the grouting oper­ation. An additional flatbed truck was used to shuttle materials from the staging area to the job site. The materials were transferred to the truck towing the grout plant where they could easily be loaded into the machine by one or two people, depending on the rate of grout pumped at the time of operation.

No absolute conclusions are evident when considering the long-term objectives. The number of years of life expectancy added to the pavement will only be known at some future date. The immediate objectives, however, were satisfied. The short­term objectives, the stabilization of the slab at the outer bound­ary where water was infiltrating and the sealing of this area by grout, were accomplished. Actual observations were made to verify this by the TDHPT. The success of this procedure will be determined, ultimately, by the TDHPT, but the results look promising. It is important to note that pavement sections that had failed were replaced, making it hard to determine what part grouting played in extending actual highway longevity. It can only be suggested that its contribution was significant. Other significant points were: (a) the scope of the project was larger than most (36 mi or 58 km), (b) it was hard to obtain an accurate approximation of grout quantities needed, (c) TDHPT estimates came within roughly 20 percent of actual quantities of grout used, and (d) no significant improvements to the esti­mating process were established as a result of this project. It is concluded that the experience gained through this project will be significant to both the TDHPT and grouting contractors when repair of this type is needed in the future.